In three-phase electric furnaces the current drawn is very high, for example, of the order of one hundred amperes, and the temperature distribution within the furnace is often very critical. For instance, the furnace may be of substantial length and may be used for heat treating and sealing of large numbers of small electronic components, integrated circuit chips, and similar products, which are slowly conveyed through the length of the furnace. The various resistance heating elements being energized by the three-phase electrical power are located at spaced positions throughout the furnace. A temperature sensor such as a thermocouple may be used to control the temperature of the furnace, but it is usually not practical to attempt to monitor the temperature in the furnace at many different points along its length.
In the event that one of the electrical heaters should fail, the temperature distribution within the furnace will change significantly, but the temperature level in the vicinity of the sensor itself may remain at the proper level since the electrical resistance heaters in the other two phases are still being energized and the sensor may be located near an energized heater. Each heater may include a number of individual heater elements connected in series, and if any one of these individual elements should fail, the entire heater in that phase will fail. As a result of this change in temperature distribution, the products may become improperly heat treated. The operator does not have any way of learning that one of the electrical phases has failed, because the furnace remains hot, and the temperature sensor may continue to show that a proper temperature level is being maintained.
However, when the products are subsequently checked by quality control personnel, which may occur several hours or even a day or two later, it becomes apparent that the furnace has malfunctioned. By this time, several thousand dollars of product inventory may have become defectively heat treated. The problem of determining when one of the resistance elements in the furnace has failed is complicated by the high-current being drawn by the load and by the fact that the load is controlled, for example, by intermittently turning the whole load on and off.
U.S. Pat. No. 3,718,920 -- Grenier shows a power control system for a polyphase power supply to the phase windings of an electric motor for giving an alarm signal if one of the phases should fail. However, this Grenier system involves the use of series resistors located in each of the three circuit legs with direct connections to opposite sides of each resistor for responding to the voltage drop thereacross. In the case of a high-current load, such as an electric furnace, it is not practical to put resistances in series with each of the heater elements because a large amount of electrical energy would be wasted by the I.sup.2 R (current squared times resistance) losses occurring in such series resistances and because of the heat dissipation problems involved with this wastage of electrical power.
U.S. Pat. Nos. 3,636,541 to Genuit et al. and 3,525,019 to Lansch are in this general field and may be of interest to the reader.